1. Field of the Invention
The present invention relates to a lens device for use in an optical apparatus such as a silver-halide camera, a video camera and a television camera, as well as to such optical apparatus.
2. Description of Related Art
In the recent trend toward further reductions in the sizes and weights of cameras such as home video cameras and photographic cameras for 35-mm film, such a camera has employed a so-called rear focus type of zoom lens as a photographing zoom lens. The rear focus type of zoom lens is characterized by the capability to readily realize a predetermined magnification variation ratio, a wide angle of view, a short overall lens length, a front lens of small diameter, and an entire lens system of small size and light weight, and is arranged to effect focusing by using a lens located on an image-plane side rather than by using a variator part. In the rear focus type of zoom lens, even in the case of the same subject, since the position of its focusing lens in the direction of its optical axis varies with the magnification varying operation of the variator part, it is necessary to adjust the position of the focusing lens in association with the magnification varying operation. To this end, various lens barrels having a mechanism for adjusting the position of a focusing lens have heretofore been proposed.
FIG. 11 is a diagrammatic cross-sectional view of the essential portion of a zoom lens barrel using a conventional rear focus type of zoom lens. The zoom lens shown in FIG. 11 includes a front lens group (a first group) 111 which is fixed, a variator lens group (a second group or variator lens) 112 which moves along the optical axis of the zoom lens to perform a magnification varying operation, a lens group (a third group) 113 which is fixed, and a lens group (a fourth group or focusing lens) 114 which moves along the optical axis for the purpose of correcting a variation in an image plane due to a magnification varying operation and effecting focusing.
The zoom lens barrel shown in FIG. 11 includes a guide rod 133 for stopping the rotation of the second group 112, a variator transporting rod 134 for moving the second group 112, a fixed tube 135, an iris unit 136 (which is inserted at right angles with the sheet surface of FIG. 11, and a focusing motor 137 such as a stepping motor. The stepping motor 137 has an output shaft 138 part of which has an externally threaded portion 138a for moving the fourth group 114. An internally threaded portion 139 is meshed with the externally threaded portion 138a and is formed integrally with a moving frame 140 of the fourth group 114. The zoom lens barrel shown in FIG. 11 also includes guide rods 141 and 142 for the moving frame 140 of the fourth group 114, a back plate 143 for positioning and pressing the guide rods 141 and 142, a relay holder 144 which holds the third group 113, a zooming motor 145, a speed reducing unit 146 for the zooming motor 145, and interlocking gears 147 and 148, the interlocking gear 148 being fixed to the variator transporting rod 134 for effecting zooming.
In the above-described arrangement, if the stepping motor 137 is activated, the fourth group 114 for focusing is moved along the optical axis by the externally threaded portion 138a formed around the output shaft 138. If the zooming motor 145 is activated, the gears 147 and 148 are interlockingly driven to rotate the variator transporting rod 134, thereby moving the second group 112 along the optical axis.
FIG. 2 is a graph showing locus data plotted against several subject distances, the locus data indicating the positional relation between the second group (variator lens) 112 and the focusing fourth group 114 with respect to the optical axis in the above-described zoom lens. In FIG. 2, there are shown in-focus positional relations against subject distances of infinity (.infin.), 2 m, 1 m, 80 cm and 0 cm by way of example. In the rear focus type of zoom lens, since the positional relation between the variator lens 112 and the focusing lens 114 differs for different subject distances as shown in FIG. 2, it is impossible to interlock the individual lens groups with each other by means of a simple mechanical structure such as a cam ring of a front focus type of zoom lens.
Accordingly, if the zooming motor 145 is simply driven in the structure shown in FIG. 11, a variation in the image plane occurs due to a magnification varying operation.
To cope with this problem, it has heretofore been proposed to provide various methods of optimally controlling the positional relation between the variator lens 112 and the focusing lens 114 such as that shown in FIG. 2, according to different subject distances.
For example, Japanese Laid-Open Patent Application No. Hei 1-280709 (corresponding to U.S. Pat. No. 4,920,369) and Japanese Laid-Open Patent Application No. Hei 1-321416 have proposed a method of tracing loci of the positional relation between a focusing lens and a variator lens with respect to an optical axis according to different subject distances.
FIGS. 12 and 13 are explanatory views of a method of maintaining the positional relation between a variator lens and a focusing lens with respect to an optical axis, which method is proposed in Japanese Laid-Open Patent Application No. Hei 1-280709.
The block diagram of FIG. 12 will be described below. The lens groups 111 to 114 are identical to those shown in FIG. 11. The position of the variator lens 112 with respect to the optical axis is detected by a zoom encoder 149. The zoom encoder 149 may be, for example, a volume encoder which is arranged so that a brush integrally secured to a variator moving ring slides a circuit board on which a resistance pattern is printed. An iris encoder 150 is provided for detecting an aperture value by using the output from a Hall element 163 provided in, for example, an iris meter. Reference numeral 151 denotes an image pickup element such as a CCD, and reference numeral 152 denotes a camera processing circuit. A Y signal of a signal obtained by the image pickup element 151 is inputted to an AF circuit 153. The AF circuit 153 determines whether a subject is in focus or out of focus, and, if the subject is out of focus, determines whether the state of focus is front focus or rear focus, as well as to what extent the subject is out of focus. The results of these decisions are inputted to a CPU 154.
A power-on resetting circuit 155 performs various resetting operations when a power source is turned on. If a zoom switch 157 is operated by an operator, a zoom operating circuit 156 transmits the contents of the operation to the CPU 154. Memory parts 154A to 154C store the locus data shown in FIG. 2, and the memory part 154A stores direction data, the memory part 154B stores speed data, and the memory part 154C stores boundary data. Reference numerals 161 and 162 denote a zooming-motor driver and a stepping-motor driver, respectively. The number of pulses continuously inputted to a stepping motor 137 from the stepping-motor driver 162 is also continuously inputted to and counted by the CPU 154, in which the obtained count value is used as an encoder value indicative of the absolute position of the focusing lens 114. In the above-described arrangement, the position of the variator lens 112 and the position of the focusing lens 114 are respectively obtained from the value outputted from the zoom encoder 149 and the number of input pulses outputted from the stepping-motor driver 162, whereby individual points on the map of the locus data shown in FIG. 2 are determined.
The map shown in FIG. 2 is divided into rectangular small areas I, II, III, . . . , as shown in FIG. 3, by the boundary data stored in the memory part 154C. In FIG. 3, each shaded portion indicates an area in which disposition of either lens is inhibited. If the location of a point on the map is determined, a small area to which the point belongs can be determined.
The rotating speed and direction of the stepping motor, which are obtained from a locus which passes through the center of each of the small areas, are respectively stored as the speed data and the direction data for each of the small areas. In the example shown in FIG. 3, the horizontal axis (the position of the variator lens 112) is divided into 10 zones. Assuming that the speed of the zooming motor is set so that the variator lens is made to move from a telephoto end T to a wide-angle end W in 10 seconds, it takes 1 second for the variator lens to pass through one zone in a zooming direction.
Referring to FIG. 13 which is an enlarged view of the area III of FIG. 3, a locus 164 passes through the center of the area III and a locus 165 and a locus 166 pass through the bottom left and the top right of the area III, respectively, and their inclinations slightly differ from one another. The focusing lens, if it moves at a speed of x mm/1 sec, can trace the central locus 164 with almost no error.
The speed of the focusing lens obtained in this manner is hereinafter referred to as an area representative speed. A plurality of area representative speeds for the respective small areas are obtained by the number of the small areas and are stored in the speed-data memory part 154B. Assuming that the locus 168 indicates such an area representative speed, the speed of the stepping motor relative to the area III is set by finely adjusting the locus 168 to the locus 167 or the locus 169 on the basis of the detection result of an automatic focus adjustment device. The direction data is stored in the direction-data memory part 154A in the form of sign data because the direction of rotation of the stepping motor varies in each of the small areas even if zooming is performed in the same direction, for example, from the telephoto end T to the wide-angle end W (or from the wide-angle end W to the telephoto end T).
In the above-described manner, if the area representative speed obtained from the positions of the variator lens 112 and the focusing lens 114 is corrected on the basis of the detection result of the automatic focus adjustment device to determine the speed of the stepping motor 137 and this speed is used to drive the stepping motor 137 during the driving of the zooming motor 145 to control the position of the focusing lens 114, it is possible to maintain an in-focus state during zooming even in the case of a rear focus type of zoom lens.
Incidentally, it is also possible to adopt a method of storing, in addition to the area representative speeds indicated by the locus 168, speeds such as those indicated by the locus 167 and the locus 169 in the speed-data memory part and selecting three speeds according to the detection result of the automatic focus adjustment device.
In addition to the above-described methods of storing the speeds, it is also possible to adopt a method of calculating a locus which passes through a point on the map, from the current position of the variator lens and that of the stepping motor and tracing the locus, or a method of previously storing a plurality of loci as the positions of the focusing lens relative to a plurality of positions of the variator lens.
Japanese Laid-Open Patent Application No. Hei 1-321416 discloses a method of previously storing the positions of a focusing lens relative to a plurality of positions of a variator lens between a wide-angle end and a telephoto end with respect to a plurality of subject distances. In this method, when zooming is started, a point which in the map is occupied by the position of the variator lens and the position of the focusing lens at that time is detected, and an interpolation computation is performed on the basis of stored data indicative of a point closest to the detected point on the front focus side thereof and stored data indicative of a point closest to the detected point on the rear focus side thereof with respect to the same focal length, thereby calculating the position of the focusing lens for the focal length (the position of the variator lens).
FIG. 14 is an explanatory view of loci for focal lengths near the telephoto end. In the art disclosed in Japanese Laid-Open Patent Application No. Hei 1-321416, regarding the loci shown in the area I of FIG. 3 (for example, an in-focus loci for infinity), information indicative of rr.sub.1, rr.sub.4, rr.sub.7 and rr.sub.9 is stored as data indicative of the positions of the focusing lens relative to positions V.sub.n (the telephoto end), V.sub.n-1, V-.sub.n-2 and V.sub.n-3 of the variator lens. That is to say, a locus LL1 which passes through points P.sub.1, P.sub.4, P.sub.7 and P.sub.10 in the map is stored as the in-focus locus for infinity. Similarly, information indicative of rr.sub.2, rr.sub.5, rr.sub.8 and rr.sub.11 is stored as the positions of the focusing lens relative to positions V.sub.n (the telephoto end), V.sub.n-1, V.sub.n-2 and V.sub.n-3 of the variator lens, that is to say, as a locus LL2 (for example, an in-focus locus for a subject distance of 10 m). In practice, similar data are, of course, prepared over the entire zoom range from the telephoto end to the wide-angle end.
If the variator lens is to be moved from a point (V.sub.n, rr), i.e., a point P in the map, points P.sub.A, P.sub.B and P.sub.C are obtained by an interpolation computation on the basis of the stored data indicative of points closest to a locus of the point P on the front focus side thereof for the respective positions of the variator lens, i.e., data on the locus LL2, and the stored data indicative of points closest to the locus of the point P on the rear focus side thereof for the same respective positions of the variator lens, i.e., data on the locus LL1. In this manner, by determining the positions of the focusing lens relative to the respective focal lengths V.sub.0 (the wide-angle end), V.sub.1, V.sub.2, . . . , V.sub.n-1 and V.sub.n (the telephoto end) which are used as the positions of the variator lens during zooming, it is possible to determine a locus to be traced by the focusing lens during zooming.
In the above description, because of the interpolation computation, the ratio of the distance between the points P.sub.1 and P to the distance between the points P.sub.2 and P becomes is equal to, for example, the ratio of the distance between the points P.sub.A and P.sub.4 to the distance between the points P.sub.A and P.sub.5.
As a matter of course, the aforesaid stored speed data or position data are created on the basis of optical design values which are determined on the assumption that manufacture error is zero.
Although in the above-described example a DC motor having a gear head is used as a zooming actuator, it is also possible to adopt a method which uses a stepping motor as the zooming actuator similarly to the case of the focusing lens. In this method, the absolute positions of the respective lens groups are detected not by using a volume encoder as a variator encoder but by counting the number of input pulses of the zooming actuator on the basis of a reset position similarly to the case of the stepping motor for the focusing lens.
There is also available a method of detecting a reference position of the focusing motor during the operation thereof by using a photointerrupter.
FIG. 15 is a perspective view showing a structure which uses photointerrupters in combination with a variator lens and a focusing lens, respectively. In FIG. 15, reference numeral 114 (112) denotes a focusing lens (or a variator lens), and parts denoted by reference numerals identical to those used in FIG. 11 have functions identical to those of the corresponding ones shown in FIG. 11. A light-blocking wall portion 201 is provided integrally with a moving frame for the variator lens 112 (or the focusing lens 114), and as the variator lens 112 (or the focusing lens 114) moves in the direction of the optical axis, the light-blocking wall portion 201 moves toward, for example, the position shown by two-dot chain lines in FIG. 15. A photointerrupter 202 includes a light emitting element and a light receiving element (neither of which is shown) which are disposed on opposite sides so that the light-blocking wall portion 201 can be inserted between both elements. Although not shown in FIG. 15, the photointerrupter 202 is fixed to a lens barrel (135 or 144 in FIG. 11).
FIG. 16 is a schematic view showing the positional relation between the light-blocking wall portion 201 and a light emitting element 203 and a light receiving element 204 of the photointerrupter. When the light-blocking wall portion 201 lies at the position shown by solid lines, the space between the light emitting element 203 and the light receiving element 204 is not obstructed. Therefore, a sufficiently large output can be obtained from the light receiving element 204. Letting L be the entire movable range of the moving frame, if the light-blocking wall portion 201 is moving in the range between L/2 and L, i.e., toward the right side as viewed in FIG. 16, the space between the light emitting element 203 and the light receiving element 204 is obstructed and almost no output is obtained from the light receiving element 204.
The position of the light-blocking wall portion 201 that is shown by two-dot chain lines in FIG. 16 represents the rightmost position of the moving frame in the movable range thereof. When the light-blocking wall portion 201 lies at that position, the space between the light emitting element 203 and the light receiving element 204 is obstructed.
FIG. 17 is a graph in which the horizontal axis represents the position of the moving frame in the direction of the optical axis, while the vertical axis represents the output of the light receiving element 204. If a threshold Th is set in this output, when the movable frame is moved in the direction of the optical axis, the output reaches the threshold Th always at a position L/2 as viewed in FIG. 17, and this position can be detected as an absolute value. Accordingly, if the absolute position of a lens group which is moving is to be detected, first, the output of the light receiving element 204 is checked when a main power source is turned on, and if the output is above the threshold Th, the stepping motor is driven to move the movable frame toward the right in this example, whereas if the output is below the threshold Th, the stepping motor is driven to move the movable frame toward the left in this example, whereby the position L/2 is detected and an address corresponding to the absolute position is assigned a predetermined numerical value. Then, driving pulses which are inputted to the stepping motor are continuously counted to detect the absolute position of the lens group which is moving.
FIG. 18 is a view similar to FIG. 2 in which the horizontal axis represents the position of the variator lens, while the vertical axis represents the position of the focusing lens, and FIG. 18 also shows the outputs of the respective photointerrupters.
As shown in FIG. 18, it is necessary to eliminate the deviation of locus information from actual positions of the two lens groups (the variator and focusing lenses) by "superimposing" the locus information on the actual positions of the two lens groups, the locus information indicating the positional relation between the two lens groups for previously stored subject distances as described previously with reference to FIG. 14 and the like. For example, in FIG. 18, a position N indicates a point at which the focusing lens can bring a subject lying at infinity into focus when the variator lens is located at the wide-angle end. Assuming that the stored locus information contains information which indicates that if the focusing lens is focused at infinity when the variator lens is set to the wide-angle end, the position of the variator lens is assigned address 100 and the position of the focusing lens is assigned address 100, an absolute address of a reset position (a position at which the output of the photointerrupter reaches the threshold Th) may be determined and written to an E.sup.2 PROM so that the N position of each of the variator lens and the focusing lens can be assigned address 100. In the example shown in FIG. 18, assuming that there is a positional difference of 1,000 pulses between the position N and the reset position of the variator lens and there is a positional difference of 400 pulses between the position N and the reset position of the focusing lens, the position N can be assigned address 100 in the case of either of the variator lens and the focusing lens if the respective reset positions of the variator lens and the focusing lens are assigned address 1100 and address 500.
Each time the power source is turned on, these moving lens groups are moved to their respective reset positions and, in this example, the reset positions of the variator lens and the focusing lens are respectively assigned address 1100 and address 500, whereby the operation of the above-described "superimposition" is completed. After that, an image forming position can be correctly held during zooming.
However, in the above-described conventional example, the writing of addresses to the E.sup.2 PROM on the basis of reset positions, such as that described above in connection with FIG. 18, is performed during an apparatus manufacturing process in a factory or the like. If the position N shown in FIG. 18, at which the focusing lens is focused at infinity for the wide-angle end, moves toward, for example, a position N' owing to a later environmental variation or the like, or if the reset positions themselves vary, the following relations will be impaired: "the position of the variator lens at the wide-angle end is assigned address 100 (in the aforesaid example)" and "the position of the focusing lens which is focused at infinity for the wide-angle end is assigned address 100 (in the aforesaid example)". That is to say, a deviation occurs in the aforesaid "superimposition". This leads to a number of problems. For example, not only does defocus occur during zooming, but also if the movable range of the focusing lens is defined with the aforesaid addresses according to the focal length (for example, the focusing lens is allowed to move only in the range of address 80 to address 150 when the variator lens is located at the wide-angle end), it becomes impossible to focus a subject lying at infinity.
This situation occurs because the positions of the lens groups are deviated by temperature or humidity variations due to expansion or shrinkage of various portions of the lens groups when an actual operating environmental temperature or humidity differs from an environmental temperature or humidity at which the writing of addresses to the E.sup.2 PROM at reset positions is performed. Otherwise, if a plastic material such as acrylic is used as a lens material, the aforesaid situation occurs not only due to such a mechanical deformation but also due to a variation in the properties, such as refractive index, of the material.
The idea of disposing a temperature sensor for correcting such a deviation of a focus position due to a temperature variation has heretofore been known. However, since the amount of focus deviation due to a temperature variation differs between individual lenses, there unavoidably remains an uncorrected deviation if a dispersion of the output gain of the temperature sensor is taken into account. The use of the temperature sensor is necessarily disadvantageous in terms of space and cost. Furthermore, in a case where a heat source such as a CCD image forming element, which is disposed in an apparatus such as a video camera, shows a temperature rise up to a considerably high temperature with time after the power source is turned on, a non-uniform temperature distribution complicatedly associated with environmental temperature occurs in either of the lens groups, so that it may become impossible to fully correct a focus deviation merely by disposing a single temperature sensor.
A focus movement (or focus deviation) is caused by a factor such as a moisture absorption, in addition to a temperature variation.
Furthermore, in the case of an interchangeable lens type of camera, when an interchangeable lens is removed from one camera body and mounted on another camera body, a focus deviation occurs irrespective of the presence or absence of a variation in temperature or humidity if there is a difference between the flange back lengths of the camera bodies.
In particular, in the field of video cameras in which the size of an image-forming image plane is becoming smaller and smaller while the number of pixels per CCD is increasing more and more, even if the differences between the flange back lengths of individual interchangeable lens type of video cameras are reduced to .+-.0.01 mm or less, it may not be possible to completely correct a slight focus deviation.
In addition, in the current trend toward further reductions in the sizes and weights of video lenses, it is desirable to omit the aforesaid reset switch.
Incidentally, the above-described focus movement due to a temperature variation differs in amount for different focal lengths. For example, if the position of the first group 111 varies in the direction of the optical axis, in general, the amount of focus movement is small on the wide-angle side and is remarkably large on the telephoto side.
The focus movement on the telephoto side, even if it occurs, has a tendency to shift focus from a locus of one subject distance toward a locus of another subject distance; for example, as viewed in FIG. 2, a locus of 1 m becomes equivalent to a locus of 2 m, and no remarkable focus deviation occurs at any intermediate zoom position. Therefore, even if the focus movement on the telephoto side is not completely corrected, if the focus movement on the wide-angle end is corrected, a remarkable defocus does not occur when zooming from the telephoto side toward the wide-angle side is performed after the completion of automatic or manual focusing for an arbitrary focal length other than the wide-angle end.
Contrarily, the position of the focusing lens for the wide-angle end is approximately the same irrespective of subject distances as shown in FIGS. 2 and 18, so that if such position deviates, defocus occurs irrespective of subject distances whenever zooming from the wide-angle side toward the telephoto side is performed.
The present invention is intended to provide a method of highly accurately correct focus deviations such as the above-described ones.